In the old legends of Krypton, Flamebird and Nightwing were powerful bird/dragon spirits who seek each others' companionship.
Nightwing and Flamebird was created by Edmond Hamilton and Curt Swan.
During Superman and Jimmy Olsen's trips to the bottled city of Kandor Superman did not have access to his powers and due to a misunderstanding, he was branded as an outlaw. To protect themselves, Superman and Jimmy disguised themselves as a crime fighting duo, Nightwing and Flamebird, named after two Kandorian species of birds.
Superman and Jimmy managed to clear their names, and were replaced as Nightwing and Flamebird respectively by Superman's cousin Van-Zee and his lab assistant Ak-Var.Due to the events of Crisis of Infinite Earths, this version of Nightwing and Flamebird was retconned out of existence.
Post-Infinite Crisis / Bottled City
After Infinite Crisis, Power Girl and Supergirl descended into the bottled city of Kandor populated by various alien races. The city had been overcome with religious fanaticism based on Superman as an idol, causing the resident Kryptonians to oppress the other alien races. This was all being led by Ultraman, who was posing as Kal-El and under the mental influence of the Saturn Queen.
Post-Infinite Crisis / New Earth
Flamebird and Nightwing are defined by their emotions. They're passion, anger, and sorrow. This is also the key to their downfall, ending in tragedy only to be reincarnated to begin the cycle over an over again. As of now, Chris Kent and Thara Ak-Var are the avatars of Nightwing and Flamebird.
Major Story Arcs
Post-Infinite Crisis / Bottled City:
The Fall of Ultraman and Saturn Queen
Taking on the masked identities of Nightwing and Flamebird, Power Girl and Supergirl became the champions of the oppressed aliens, battling against Ultraman's regime. They were both revealed as Kryptonians and captured, with Saturn Queen trying to brainwash them to serve as part of the mentally controlled royal family she was trying to build for herself. However, she was unable to control them. They broke free and brought down the empire she had built around Ultraman.
Post-Infinite Crisis / New Earth:
When the true city of Kandor was restored, a mysterious new Nightwing and Flamebird appeared. They prevented Commander Gor and other loyalists from freeing General Zod from the Phantom Zone. They could not stop that from happening indefinitely, though. General Zod was freed when New Krypton was created in Earth's solar system, and the Kryptonian duo took on a new mission.
The truth was that, respectively, Flamebird and Nightwing were Thara Ak-Var and Christopher Kent. Chris' captivity in the Phantom Zone allowed him to discover a conspiracy of sleeper agents General Zod had sent to Earth, and Thara decided they would stay on Earth to topple Zod's plans before he succeeded in sparking a war between humans and Kryptonians. Because of the anti- Kryptonian sentiment on Earth, they altered their costumes to look like power suits to explain away their powers.
Their Identities Exposed to Public
As their activities began, General Zod realized their true identities and dispatched Ursa to stop them. They also got the attention of General Sam Lane, who became suspicious and began an investigation of them. For a time, this distracted the duo from their primary mission against Zod's sleeper agents. General Lane's attack on them left them exposed to the public as Kryptonians and framed for a terrorist attack on Metropolis' water system.
The Kryptonian Couple
Distractions were present internally as well. Thara's belief that she was the living avatar of the Flamebird became evident when the Flamebird began possessing her in times of great stress. Chris, however, did not truly believe he was the Nightwing's avatar. He simply lacked Thara's religious faith. On top of that, Chris continued to experience bursts of accelerated aging, pushing him to the point of becoming an elderly man. The Kryptonian couple unknowingly sought the help of Jax-Ur, a sleeper agent posing as a scientist at S.T.A.R. Labs, to fix Chris' aging problem.
Unknown to either of them, the Flamebird entity had possessed Thara and made a deal with Jax-Ur to save Chris' life. In return for his efforts, he was allowed to take a DNA sample of Thara while the Flamebird was in full possession of her body.
Powers & Abilities
Chris and Thara are both pure-blooded Kryptonians and possess superhuman levels of strength, speed and perception as well as flight, heat vision and freeze breath. However, Chris' birth in the Phantom Zone came with genetic anomalies. Though still superhumanly strong, he is not as strong as average Kryptonians. On the other hand, he has developed powers of tactile telekinesis. Thara is as strong as the average Kryptonian in yellow sunlight, and she is a trained member of the Military Guild. As a result of being the avatar of the Flamebird, she has shown the ability to generate and control fire when possessed by the Flamebird.